Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand inkjet, individual droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include thermal bubble formation (thermal inkjet (TIJ)) and piezoelectric transducers. In another process known as continuous inkjet (CIJ), a continuous stream of droplets is generated and expelled in an image-wise manner onto the surface of the image-recording element, while non-imaged droplets are deflected, caught, and recycled to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
Ink compositions containing colorants used in inkjet printers can be classified as either pigment-based, in which the colorant exists as pigment particles suspended in the ink composition, or as dye-based, in which the colorant exists as a fully solvated dye species that consists of one or more dye molecules. Pigments are highly desirable since they are far more resistant to fading than dyes. However, pigment-based inks have a number of drawbacks. Great lengths must be undertaken to reduce a pigment particle to a sufficiently small particle size and to provide sufficient colloidal stability to the particles. Pigment-based inks often require a lengthy milling operation to produce particles in the sub-micron range needed for most modern ink applications. If the pigment particles are too large light scattering can have a detrimental effect on optical density and gloss in the printed image.
A second drawback of pigmented inks is their durability after printing, especially under conditions where abrasive forces have been applied to the printed image. Pigment-based inks typically reside at the surface of the imaging receiver to which they are printed and this makes the printed images particularly susceptible to abrasive forces. To this extent, pigmented inks have been formulated with various polymer binders, dispersants and other addenda to provide durable images that can withstand post printing physical abuse and environmental conditions.
The degree of abrasion resistance of a printed image is also a function of time after printing. At short time intervals after printing, typically from a few minutes to a few hours, the ink undergoes several complex dynamic changes. As the ink contacts the receiver, some of the components penetrate into the receiver and the droplets can simultaneously spread laterally on the receiver surface. Carrier fluids such as water and humectants are drawn into the receiver by capillary forces and the polymer binders begin to film form. At short time intervals the binder film formation is incomplete and the resulting pigment cake is particularly susceptible to abrasive forces. In some cases, the incomplete polymer binder film formation results in a tacky surface that can stick to surfaces within the printer that transport the printed image. Typically, the more total fluid that is printed to the receiver (and hence more water) the longer it takes for the ink to dry and form a durable image.
The abrasion resistance of the image is further affected by the presence of humectants, which are necessary for optimal firing performance, but which are retained in the pigment cake for some period of time. Since most humectants have much lower vapor pressures than water, they are relatively slow to evaporate and can be retained in the image receiver for several hours. Humectants can have the effect of plasticizing the polymer binder and making the surface of the image tacky or softer than if no humectant was present. Once the humectants evaporate, the resulting pigment cake, consisting primarily of pigment and binders, reaches a steady state composition and determines the long-term abrasion resistance of the printed image.
Images printed from an inkjet printer are also susceptible to abrasive forces as the image receiver is advanced through the printer. Typically, there is some mechanical means, such as a series of transport rollers, for advancing the print past the printhead and out of the printer. In some printer designs a spur wheel is used to advance the printed receiver. Spur wheels are often made from a hard plastic or metal and have the shape of a disk with points or spurs located on the periphery of the wheel. The spurs contact the printed receiver and can physically penetrate the uppermost area of the printed image leaving behind a small hole. In extreme cases the spurs can plow into the receiver and tear off small sections of the imaged areas. In either case, the mechanical abrasion caused by the spur wheel occurs at short time intervals on the order of a few seconds after printing and results in a defect that is objectionable to the eye.
Pigmented inks for inkjet printing have been formulated with acrylic polymers; however, the acrylic polymers alone are insufficient in providing durable images that resist scratches and other forms of physical abuse. A second class of polymers that have been used as abrasion resistance additives in pigment-based inks are the polyurethanes, or urethane resins as they are sometimes called. U.S. Pat. No. 6,136,890 discloses a pigment-based inkjet ink wherein the pigment particles are stabilized by a polyurethane dispersant. United States Publication Number 2004/0242726 discloses a pigment dispersed by a cross-linking step between a resin having a urethane bond and a second water-soluble polymer.
Although polyurethanes are known for their excellent abrasion resistance, they also have a number of drawbacks. For example, not all polyurethane polymers are conducive to jetting from a thermal inkjet head. In particular, water-dispersible polyurethane particles, such as those disclosed in U.S. Pat. Nos. 6,533,408 and 6,268,101, Statutory Invention Registration Number US H2113H, and United States Patent Application Publication Numbers 2004/0130608 and 2004/0229976 are particularly difficult to jet from a thermal inkjet printhead at high firing frequencies. It is highly desirable to fire inks at high firing frequencies from an inkjet printer since this is one variable that controls the speed at which the image can be printed.
Another way to improve the abrasion resistance of a printed image is to apply a clear ink as an overcoat to the image. The clear inks, also known as colorless ink compositions, are typically formulated with polymer, water, and other components commonly used in aqueous-based inkjet ink formulations, for example, humectants, organic solvents, surfactants and biocides. United States Patent Application Publication Numbers 2006/0100306 and 2006/0100308 disclose the use of polyurethanes and mixtures of polyurethanes and acrylic polymers having specified acid numbers for use in clear ink compositions. However, clear inks formulated with polyurethanes also suffer from the same short term durability issues as colored inks since they have many components in common with their colored ink counterparts. In addition, the application of a clear ink increases the total amount of water applied to the receiver and therefore slows down the drying of the imaged area of the prints. Although the application of clear ink can improve the long term durability, its application can adversely affect the short term durability due to the increased water load on the receiver.
Both pigment and clear inks can be difficult to jet through inkjet print heads having small nozzle diameters especially by the thermal inkjet printing process. In recent years, thermal inkjet printers have moved to higher jetting frequencies and smaller nozzle diameters to provide faster printing speeds with higher image quality. Thermal inkjet printers are now capable of printing (in drop volumes of 3 picoliters or less) at jetting frequencies in excess of 10 kHz and the need for higher velocity firings is a highly desirable feature. However, this high frequency firing often comes at the cost of variability in the firing velocity, which leads to poor image quality in the final printed image. In addition, the demands of current thermal inkjet printing require that the nozzles fire for a large number of firings during the life-time of a printer. As an example, a typical inkjet nozzle may be required to fire in excess of 5×107, and up to as many as 1×109, individual firing events without malfunctioning or ceasing to fire altogether.